1. Field of the Invention
The present invention generally relates to image forming apparatuses, image processing programs interpreted and executed by a computer provided in the image forming apparatus, and storage media storing the image processing programs, and more particularly to a technique of improving granularity and moiré phenomenon of an output image when outputting, from an image output apparatus such as a printer, a digital image obtained by inputting a printed document to an image reading apparatus such as a scanner, for example.
2. Description of the Related Art
For example, in a case where a digital image, which is obtained by inputting a printed document to an image reading apparatus such as a scanner, is displayed on a display or output from an image output apparatus such as a printer, when no process is performed on the digital image or if performed, only image processing not relating to the image quality, such as scaling and rotation, is performed, there is a possibility that degradation of a reproduced image occurs such as occurrence of moiré phenomena and degradation of granularity.
When a halftone area of the printed document is formed with a dot pattern generated through applying screening (such as the dither method), the error diffusion method or the like, the above-described phenomena occur due to lack of consistency between such image processing and the image processing such as a halftone process performed in the image output apparatus, a printer, for example.
In order to improve the image quality by avoiding such problems of image degradation, the halftone area expressed with the dot pattern should be converted into continuous tone expression. Such process for improving the image quality is conventionally embedded in an image apparatus such as a digital multi-functional apparatus. However, it should be noted that important edges, graphics such as letters and line work, and the like must not be gradated.
As for related prior art, there are Japanese Laid-Open Patent Applications No. 6-62230, No. 10-208038, No. 11-41469, and No. 2001-211327.
Japanese Laid-Open Patent Applications No. 6-62230 and No. 10-208038 disclose techniques of separating a letter area and an image area, and performing a linear filter process having a different frequency characteristic for each pixel according to the image characteristics surrounding the pixel, thereby converting the halftone area into continuous tone expression.
Additionally, in Japanese Laid-Open Patent Applications No. 11-41469 and No. 2001-211327, methods of using the wavelet transform are devised.
However, with the recent growing popularity of network environments, a digital image input through an image reading apparatus is sent to a distant place via a network, and a recipient edits/reuses/prints out/resends the image input through the unknown image reading apparatus on a personal computer, for example. In such a network environment, new technical problems that did not exist in the past arise.
Further, it should be noted that the techniques disclosed in the above-described Japanese Laid-Open Patent Applications No. 6-62230, No. 10-208038, No. 11-41469, and No. 2001-211327 fail to consider such a network environment.
The technical problems arising in the network environment are as follows.
(1) To achieve a practical processing speed even in a case where an image input through an unknown image reading apparatus is edited/reused/printed out/resent by a personal computer, for example, and the functions for improving granularity and moiré phenomena of the output image are realized by software that is installed in or interpreted by a personal computer.
In the network environment, an image input at a distant place and sent via a network is subjected to an image quality improving process on the personal computer and then often saved as a file, output by a printer, and even sent via the network. In this case, when such functions are realized by software that is installed in or interpreted by the personal computer, it is difficult to achieve a practical processing speed. For example, when the above-described linear filter and wavelet transform are realized by software and applied to an A4 size full-color document image, the computation time becomes impractical as the resolution is increased.
Therefore, it is desirable that a practical processing speed be achieved even when the functions described above are realized by software.
(2) To attenuate the difference in the characteristics between an image processing apparatus on a sender's side of an image and an image processing apparatus on a recipient's side of the image.
In a stand-alone image processing apparatus, image processing algorithms and parameters are often designed according to the characteristics of the apparatus. However, an image processing system optimized for a specific model does not always exert the effect on models having different characteristics. In the network environment, image processing apparatuses, image input apparatuses such as a scanner in particular, possess various characteristics. Further, when an image is sent from a distant place, the characteristics may even be unknown. Therefore, in a case where an image input by an image input apparatus on the sender's side of the image and including correction information depending on the characteristics of the image input apparatus is subjected to a reproduction process by an image processing apparatus that is on the recipient's side of the image and possesses different characteristics from those of the sender's side image input apparatus, or where the reproduction process is performed on an image including unknown correction information, the image quality to be reproduced may be significantly degraded, which may result in degradation of the granularity and occurrence of moiré phenomena of the output image.
Accordingly, even in such a case where an image input by an image input apparatus on the sender's side of the image and including correction information depending on the characteristics of the image input apparatus is subjected to a reproduction process by an image processing apparatus that is on the recipient's side of the image and possesses different characteristics from those of the sender's side image input apparatus, or where the reproduction process is performed on an image including unknown correction information, it is desired to easily apply an image quality improving process that improves the granularity and moiré phenomena of an output image so as to obtain a satisfactory image quality.
(3) To realize image reproduction with a good image quality on the recipient's side of the image without depending on details (types, kinds) of a halftone processing method performed on the-sender's side of the image.
When the method of the halftone process generating the dot pattern and the parameters (threshold value matrix of dither, filter factors of error diffusion, screen angles, dot cycles and the like) are recognized, it is possible to use the method specialized therefor when performing the image reproduction process in an image processing apparatus. However, in a case where targets are document images printed using unknown processes such as printed books, magazines and newspapers, when the sensitivity for high frequency of the image input apparatus is not adequate or the resolution is low, there is a possibility that the features of the halftone pattern cannot be extracted.
Accordingly, it is desired to realize image reproduction with a good image quality by improving the granularity and moiré phenomena of the output image on the recipient's side of the image irrespective of a processing method used in the halftone processing method of the image input by the image input apparatus on the sender's side of the image.